This invention refers to static transistor switches for d.c. loads with high turn-on current, and, more specifically, to a control circuit for this type of switch.
We know that some electrical and electronic devices, such as incandescent lamps and induction motors, require a turn-on current that is much greater than the operating current. To connect a device of this type to a d.c. power source by means of a control signal issuing from a lower power signal generator, a switching transistor is customarily used said transistor, connected in series with the device itself and controlled at its base, either directly or indirectly, by the control signal. If the power of the control signal is insufficient to directly control the base of the transistor so as to produce switching from a cutoff state to a saturation state, one or more cascaded common-emitter transistors are connected before the switching transistor to amplify the power of the control signal. Obviously, since the current amplification requirements are different for turn-on and operating conditions, the control circuit must be designed for the turn-on conditions. This brings about a needless dissipation of energy during normal operating conditions. In many cases, a pair of transistors interconnected in the Darlington configuration are used in place of the switching transistor to eliminate this inconvenience. Although this solution eliminates all or part of the cascade amplifying transistors due to the improved current gain of the Darlington pair, and thus reduces dissipation in the control circuit, it provides a lesser degree of useful voltage on the load than that had by using a single switching transistor, assuming an equal supply voltage. In fact, we know that a Darlington pair has a much greater saturation voltage than the single transistor.